December 2005 LIP of the Month

Corresponds to part of event #31 in LIP record database.

Mafic volcaniclastic deposits in the Ferrar LIP, Antarctica

Pierre-Simon Ross ⁽¹⁾ (p_s_ross@hotmail.com) and James D.L. White ⁽¹⁾ (james.white@stonebow.otago.ac.nz)

⁽¹⁾ Department of Geology, University of Otago, PO Box 56, Dunedin, New Zealand;
http://www.otago.ac.nz/geology/students/psross/psross.html

In relatively young continental flood basalt provinces such as the Deccan Traps or the Columbia River Basalts, the lower part of the volcanic stratigraphy and the subvolcanic rocks are not well exposed because the flood lavas can form km-thick piles. In older provinces such as the Ferrar LIP in Antarctica, the lavas have been partly eroded, so the early stages of the eruptions – preserved as mafic volcaniclastic deposits (MVDs) – can be studied. MVDs are also known from several other LIPs and these occurrences were recently reviewed by Ross et al. (2005).

The Ferrar LIP belongs to a group of Jurassic magmatic provinces which also includes the Karoo (Southern Africa, with correlatives in the Weddell Sea sector of Antarctica) and the Chon Aike (South America and Antarctic Peninsula). The Chon Aike consists mostly of silicic volcanic rocks (http://www.mantleplumes.org/SLIPs.html) and is only partly contemporaneous with the Karoo and Ferrar, which are dominated by tholeiitic dolerite sills and mafic lavas (Elliot, 1992; see this link for an update of Ferrar petrology). The combined Karoo and Ferrar provinces have an estimated original volume of ~1.5 x 10⁶ km³ (Eldholm and Coffin, 2000).

Some 98% of Antarctica is covered by ice, but excellent outcrop exists in the Transantarctic Mountains, where the Kirkpatrick flood basalts (Ferrar LIP) occur in three distinct areas (see ‘v’ symbols on Fig. 1). The preserved thickness of lavas varies from 380 m to 780 m, and the basalts are everywhere underlain by MVDs ranging in thickness from 10 to >400 m (Hanson and Elliot, 1996; Elliot and Hanson, 2001; Fig. 2). In South Victoria Land at least, the mafic clasts in the fragmental deposits (referred to as the ‘Mawson Formation’) belong to the same magma type than the bulk of the Kirkpatrick Basalts (Ross, 2005). Most of the explosive activity in the Ferrar LIP occurred because of violent magma-groundwater interaction (Hanson and Elliot, 1996; Elliot and Hanson, 2001; White and McClintock, 2001), so the temporal evolution from explosive activity to eruption of lava flows is not thought to be due to a difference in magma chemistry.

The best and most extensive exposures of the Mawson Formation are at Coombs Hills and Allan Hills. At the former site, MVDs are interpreted to fill a large volcanic vent complex, also known as a ‘phreatocauldron’ (White and McClintock, 2001) because of the inferred phreatomagmatic fragmentation mechanism and the overall cauldron shape of the complex (Fig. 3). In other words, much of Coombs Hills represents a partly eroded volcano and we can have a look at the conduits through which explosive eruptions of the Ferrar LIP threw debris toward the paleo-surface. The conduits are preserved as cross-cutting volcaniclastic bodies in the vent complex (Ross and White, 2005a). The pipes filled with country rock-rich lapilli-tuff or tuff-breccia (Figs. 4, 5) are interpreted to have formed following phreatomagmatic explosions occurring near the walls or floor of the vent complex, causing fragmentation of both magma and abundant country rock material. In contrast, some of the cross-cutting zones filled with basalt-rich tuff-breccia or lapilli-tuff could have been generated following explosions taking place within pre-existing basalt-bearing debris, well away from the complex walls or floor. Other basalt-rich zones, accompanied by domains of in situ peperite and coherent basalt, are inferred to have originated by less violent processes.

Following the development of the vent complex at Coombs Hills, coarse unbedded MVDs were invaded by mafic plugs, some more than 150 m-wide (Fig. 6). These plugs could represent feeders to the lavas higher up in the sequence, as their diameters are more than sufficient to accommodate the inferred mass eruption rates for continental flood basalts and their chemistry matches that of the lavas (Ross, 2005). Both the plugs and the older MVDs were then cross-cut by volcaniclastic dikes up to 75 m-wide. Such clastic dikes are among the largest exposed anywhere on earth and we infer that the ash-grade material filling them was obtained by mobilization and sorting/filtering of the coarse vent-filling debris by fluidization above large basaltic intrusions (Ross and White, 2005b).

A few km away across a glacier at Allan Hills, the Mawson Formation can be divided into two members, m₁ and m₂ (Reubi et al., 2005). Member m₁ is exposed only at central Allan Hills, consists essentially of sedimentary material from the underlying Beacon Supergroup, and is interpreted as a 180 m-thick debris avalanche deposit (see Reubi et al., 2005, for details – as far as we are aware, this is the first report of a debris avalanche deposit in a LIP setting). Member m₂, which is exposed at both central and southern Allan Hills, consists predominantly of metre-thick mafic volcaniclastic layers that fall into three broad categories: (a) poorly sorted, coarse lapilli-tuff and tuff-breccia (Fig. 7); (b) block-rich layers (Fig. 8); (c) tuff and fine lapilli-tuff. The former type is interpreted as the deposits of high-concentration pyroclastic density currents, probably formed during the collapse of phreatomagmatic eruption plumes (Ross and White, 2005c). Occasional block-rich layers probably were formed by both ballistic fall from local vents and pyroclastic flows, and the finer-grained layers were probably deposited by dilute currents. Dilute, moist turbulent pyroclastic density currents were also likely responsible for the generation and deposition of large (≤4.5 cm) rim-type accretionary lapilli (Fig. 9). It is inferred that all these layers from the m₂ member were deposited in a pre-existing topographic depression.

The presence of surface water is easily demonstrable at Carapace Nunatak, to the south of Allan Hills. There, sandstones correlated with the Mawson Formation were laid down on an alluvial plain (Ballance and Watters, 1971). Then, after deposition of finer grained lacustrine sediments and emplacement of a first lava (Bradshaw, 1987), a 40 m-thick unit comprising pillow lavas and pillow-palagonite breccias was formed (Fig. 10). The overlying lavas were likely subaerial, indicating the lake had dried-out or had been filled by coherent lava and fragmented lava at this stage.

In summary, MVDs of the Ferrar LIP predominantly consist of coarse lapilli-tuffs and tuff-breccias up to 400 m-thick which either fill self-generated composite holes-in-the-ground (phreatocauldrons) or pre-existing topographic depressions. They underlie the Kirkpatrick flood basalts throughout the Transantarctic Mountains and represent the phreatomagmatic precursors to flood volcanism. Surface water and groundwater played a major role in controlling the eruption styles. The potential for MVD-forming eruptions to generate significant environmental impacts (perhaps even causing mass extinctions) remains to be evaluated. As mentioned by Ross et al. (2005), apart from working on the physical volcanology of the MVDs to assess these potential impacts, one would also need to (1) acquire more data on the pre-eruptive volatile contents of the involved magmas; and (2) model eruption plumes for individual eruptions of known volume and dispersal, and in particular the height of release of volatiles in the atmosphere.

References